1. Field of the Invention
The present invention relates to acoustic transducers. More particularly, the present invention relates to an acoustic sensor and projector module for sonar applications.
2. Description of Related Art
Sonar search systems employ arrays of acoustic transducer modules to sense pressure wave returns from underwater targets. Acoustic transducer modules include oscillatory transducer elements that project acoustic beams when electrically excited, and which generate electrical signals in response to acoustic energy. Thus, such a transducer element can be used in sonar systems as either a projector for generating acoustic beams, or as a hydrophone for sensing acoustic wave returns. However, an acoustic transducer element cannot simultaneously be used both as a hydrophone and as a projector.
To sense acoustic signals at selected wavelengths, sonar transducer modules include acoustic baffles that enable the transducer element to receive signals at the desired wavelength, but which attenuate, or "baffle," unwanted noise. One type of baffling arrangement is a 1/4 wave baffle design comprising layers of high and low acoustic impedance materials surrounding three sides of the acoustic transducer element. The thickness of each layer is selected to correspond to a 1/4 wavelength of sound, as determined by the speed of sound within the particular material.
The layered baffle structure provides a very lossy composite baffle that serves to attenuate unwanted acoustic noise from all directions other than the desired one, significantly increasing the directivity index (DI) of the acoustic transducer element. By carefully selecting the dimensions of each of the layers, the baffle can be configured to reflect the desired signal back in phase to the transducer element. The reflected signals improve the sensitivity of the transducer element by constructively adding to the signal. At the same time, the unwanted noise can be made to cancel itself, thereby further reducing the noise level of the transducer element.
Most side looking search sonar systems require two distinct arrays of acoustic transducer modules dedicated to projecting and sensing, respectively. In sonar systems of this type, one set of acoustic transducer modules forms a projector array that directs an acoustic beam at the target, and another group of acoustic transducer modules serves as a hydrophone array for sensing pressure waves returned from the target. The projector and hydrophone arrays are commonly housed together in a common metal frame that is sealed and filled with an acoustic fill fluid. Acoustic transducer modules having the 1/4 wave baffle design can be used to form both the projector and hydrophone arrays. However, the baffle structure increases the weight and volume of each module and, consequently, the weight and volume of each array of modules.
Increasingly aggressive mission requirements for sonar search systems have led to significant efforts to reduce the weight, volume, and power consumption of the overall sonar system. One possible way to achieve the desired reductions is to modify the design of the acoustic transducer modules. For example, a sonar system has been proposed that eliminates the need for distinct projector and hydrophone arrays, using a single array of modules for both projecting and sensing.
This technique, commonly called transmit/receive (T/R) switching, incorporates electronic circuitry that switches the individual transducer elements in a single array of modules between projecting and sensing modes of operation. However, the additional circuitry required by the T/R switching design undesirably increases the complexity of the overall sonar device. Moreover, the weight and volume added by the electronic components negate the savings achieved by elimination of the additional array of transducer modules. Accordingly, the need remains for a transducer module design realizing the goals of reduced weight and volume.